The present invention relates to a cathode ray tube having an electron gun equipped with a main lens having a function of controlling a shape of an electron beam spot which is deflected to the peripheral portion of an display screen, to improve a resolution at the peripheral portion of the screen of the cathode ray tube for use in a direct view color television receiver or a color display terminal.
The cathode ray tube which is utilized in color display of a direct view type or projection type television receiver, display terminal device and the like, is composed of a panel portion that is an image screen, a neck portion accommodating an electron gun, and a funnel portion for connecting the panel portion and the neck portion. A deflection yoke is attached to the funnel portion for scanning an electron beam emitted from the electron gun on a phosphor screen that is formed on an inner face of the panel portion.
The electron gun which is accommodated in the neck portion is provided with an electron beam generating unit having a cathode for generating the electron beam and a control electrode for controlling the electron beam, and a main lens unit comprising various electrodes for focusing, accelerating and converging the controlled electron beam.
The electron beam emitted from the cathode is modulated by signals applied on the control electrode or the cathode, and is directed onto the phosphor screen after being formed into a required sectional shape and provided with a required energy by the main lens electrodes.
FIG. 5 shows a schematic sectional diagram for explaining an example of the structure of the color cathode ray tube, of which shape of the electron gun portion is exaggerated for the purpose of explanation.
In FIG. 5, the electron gun accommodated in the neck portion is composed of the electron beam generating unit and the main lens unit which accelerates and focuses the electron beam generated from the electron beam generating unit and the electron beam is made to impinge on a phosphor screen 3 composed of three color phosphor materials which are coated and formed on an inner wall of a faceplate portion 2 composing a glass envelope 1.
The electron beam generating unit is composed of cathodes 7, 8 and 9, a first grid electrode (G1)10, and a second grid electrode (G2) 30. The electron beams which have been emitted from the cathodes 7, 8 and 9, are radiated along center axes 35, 36 and 37 which are disposed approximately in parallel with each other in a common plane (in the horizontal direction) and are incident on the main lens unit after passing through the first grid electrode 10 and the second grid electrode 30.
The main lens unit is composed of a third grid electrode (G3) 31 that is one main lens electrode, a fourth grid electrode (G4) 32 and a shield cup electrode 33. The center axes of electron beam passing holes 70, 71, 72, 76, 77 and 78 which are formed in the third grid electrode (G3) 31 and the shield cup electrode 33, are on the center axes 35, 36 and 37, respectively.
Further, the center axis of a central electron beam passing hole 74 of the fourth grid electrode 32 which is the other main lens electrode, is on the center axis 36. However, the center axes 38 and 39 of side electron beam passing holes 73 and 75 are not on the center axes 35 and 37, and are slightly displaced from the center axes 35 and 37 toward the outside, respectively.
In operation, the potential level of the third grid electrode 31 is set lower than that of the fourth grid electrode 32. The fourth grid electrode 32 and the shield cup electrode 33 having a high potential level is connected to a conductive film 5 such that the potential level thereof is equal to that of the conductive film 5 that is coated on the inner face of the funnel portion by a conductive spring or the like, not shown.
Since the center electron beam passing holes of the third grid electrode 31 and the fourth grid electrode 32 are coaxial, an axisymmetric main lens is formed at the central portions of the two electrodes, and the central electron beam is focused by the main lens and proceeds straight on a trajectory along the axis.
On the other hand, since the axes of the side electron beam passing holes of the two electrodes are deviated from each other, a non-axisymmetric main lens is formed at the side. Therefore, the outside electron beams pass through locations which are deviated from the center axes of the lens toward the central electron beam in a diverging lens region that is formed on the side of the fourth grid electrode 32, in the main lens region, and receive a focusing action by the main lens and at the same time a converging force toward the central electron beam.
In this way, the three the electron beams are focused and at the same time converged on a shadow mask 4 to be overlapped. This converging action is called a static convergence.
The electron beam receives a color selection at an opening of the shadow mask so that only a portion thereof passes through the opening to excite a phosphor of a color corresponding to the respective electron beam.
Further, the deflection yoke 6 deflects and scans the electron beam on the phospher screen in the horizontal and vertical directions thereby forming a two-dimensional image on the phosphor screen.
Conventionally, an electron gun for a color picture tube having a so-called electrostatic quadrupole lens has been proposed to improve a resolution at a peripheral portion of the screen.
In the electron gun of this type, the cathode, the first grid electrode and the second grid electrode compose the electron beam generating unit, a plurality of electron beams are emitted from the electron beam generating unit along initial paths which are arranged approximately in parallel with each other in a horizontal plane, and are incident on the main lens unit composed of the focusing electrode, the accelerating electrode and the shield cup electrode.
The focusing electrode composing the main lens unit is composed of a first member and a second member, and the electrostatic quadrupole lens is composed by opposing an aperture electrode provided in the first member and planar correction electrodes provided in the second member.
The acceleration electrode is impressed with a final accelerating voltage of 20 through 35 kV that is the highest voltage. Further, a first focusing voltage is applied on the focusing electrode, which is normally a constant voltage of 5 through 10 kV.
On the other hand, a second focusing voltage is applied on the second member of the focusing electrode. The second focusing voltage comprises a constant voltage superposed by a dynamic correction voltage that changes in synchronism with a deflection amount of the electron beam.
The resolution at the peripheral portion of the screen of a color cathode ray tube is considerably improved by using the above electron gun. That is, a correction is performed wherein an astigmatism which elongates in the horizontal direction the electron beam spot that is deflected to the peripheral portion of the screen owing to a self-convergent magnetic deflection field and another astigmatism that elongates the electron beam formed by the electrostatic quadrupole lens in the vertical direction cancel each other.
The distance from the main lens to the center of the screen and the distance from the main lens to the peripheral portion of the screen are different. Therefore, when the electron beam is focused at the center of the image plane in an optimum condition, the focusing condition is deviated from the optimum condition at the peripheral portion of the screen, and this is a curvature-of-field aberration which brings about the deterioration in the resolution. The curvature-of-field aberration is corrected by the above-mentioned dynamic correction voltage, voltage is applied, the intensity of the main lens which is a final stage lens formed between the accelerating electrode and the second member of the above-mentioned focusing electrode, is reduced, the deflected electron beam can be optimally focused at the peripheral portion of the screen, and the curvature-of-field aberration as well as the astigmatism are corrected.
However, when the electron gun having this electrostatic quadrupole lens is employed, an electric circuit for generating the dynamic correction voltage is necessary, which increases the production cost especially when the dynamic correction voltage is high. Accordingly, it is necessary to improve a correction sensitivity in deflection aberration.
When the strength of the electrostatic quadrupole lens is increased, the correction sensitivity of the astigmatism in the deflection aberration can easily be improved. However, with respect to the curvature-of-field aberration, the correction sensitivity can not be easily improved, since the curvature-of-field aberration is corrected by the main lens. When the strength of the main lens is increased to improve the correction sensitivity for curvature-of-field aberration, it is not possible to focus the electron beam on the screen, even when the electron beam is not deflected.
Even when the correction sensitivity with respect to only the astigmatism is improved, an unbalance thereof with a curvature-of-field correction is caused which does not result in the reduction of the dynamic correction voltage.
Accordingly, a structure of an electron gun for reducing the dynamic correction voltage and reducing the production cost has been proposed.
FIG. 6 is a schematic diagram for explaining a structure of an electron gun for improving the correction sensitivity in the astigmatism at a low cost without reducing the correction sensitivity for curvature of field, wherein numeral 8 designates a cathode, numeral 10 designates a first grid electrode, numeral 30 designates a second grid electrode, numeral 31 designates a focusing electrode group composing a third grid electrode, numeral 32 designates a fourth grid electrode composing an accelerating electrode, and numeral 33 designates a shield cup electrode.
As shown in FIG. 6, the focusing electrode 31 is divided into a plurality of electrode members 31-1, 31-2, 31-3, 31-4, 31-5 and 31-6. Among the members of a focusing electrode group, in addition to an electrostatic quadrupole lens, at least one axisymmetrical lens is provided which has a function of a curvature-of-field correction lens. Further, the main lens is provided with a strong astigmatism which deforms the sectional shape of the electron beam into the vertically elongated shape. On this occasion, it is necessary to change direct voltage components of two focusing voltages in the above-mentioned conventional electron gun. However, the method of applying the dynamic correction voltage remains the same.
That is, in the conventional gun, the two direct focusing voltages are approximately the same value, and the dynamic correction voltage increases with an increase in the deflection amount of the electron beam. On the other hand, in the electron gun shown in FIG. 6, one of the two direct focusing voltages is considerably made larger than the other, and the difference in voltages is at least larger than the maximum value of the dynamic correciton voltage. In this way, the difference in potential in the axisymmetric lens is reduced and the strength of lens is also reduced when the deflection amount of the electron beam and therefore the dynamic correction voltage increase.
Accordingly, a force for focusing the electron beam is weakened in deflecting the electron beam thereby correcting the curvature-of-field aberration.
In this way, at least one curvature-of-field correction lens is added to the conventional curvature-of-field correction lens that is conventionally provided with only the main lens. Therefore, it is possible to reduce the dynamic correction voltage.
Further, it is possible to reduce a voltage necessary for correction, also with respect to the correction of the astigmatism, by increasing the intensity of the electrostatic quadrupole lens or by increasing the number thereof.
In this way, in the color cathode ray tube employing the electron gun of the type shown in FIG. 6, the dynamic correction voltage can be reduced and the increase in the cost of the circuit can be restrained.
The electron gun employing the above electrostatic quadrupole lens has been disclosed in Japanese Laid Open Patent Publication No. 43532/1992.
However, in the color cathode ray tube employing the electron gun disclosed in the Japanese Laid Open Patent Publication No. 43532/1992, there is the following problem owing to the structure of electrodes of the electron gun.
The effect of correction for curvature of field by the above axisymmetric lens is weak in comparison with the effect by the main lens. Therefore, the focusing electrode should be divided into a number of electrodes and a number of, or actually 4 or 5 axisymmetric lenses should be formed to considerably reduce the dynamic correction voltage.
This brings about a complicated structure of the electron gun and the requirement for the accuracy in manufacturing it is very severe.